Volumetric change phenomena have been observed in permanently crosslinked polymer gel networks. As an external environmental condition (e.g., temperature; solvent composition; pH, electric field; light intensity and wavelength; pressure, ionic strength) is changed, the polymer network contracts and/or expands in volume. The volume of such a gel may, under certain circumstances, change reversibly by a factor as large as several hundred when the gel is presented with a change in external conditions (i.e., the gel is a "responsive" gel). Tanaka, Physical Review Letters, Vol. 40, no. 12, pp. 820-823, 1978 and Tanaka et al, Physical Review Letters, Vol. 38, No. 14, pp 771-774, 1977; Tanaka et al Physical Review Letters 5, Vol 45, pg. 1636, 1980; Ilavsky, Macromolecules, Vol. 15, pg. 782, 1982; Hrouz et al, Europ. Polym. J., Vol. 17, pg. 361, 1981; Ohmine et al, J. Chem. Physics, Vol. 8, pg. 6379, 1984; Tanaka et al, Science, Vol. 218, pg. 462, 1972 and Ilavsky et al, Polm. Bull. Vol. 7, pg. 107, 1982, all of which are incorporated herein by reference.
In gels that are conventionally made by copolymerization/cross linking reactions, such as gels comprising poly(N-isopropylacrylamide)(PNIPAAm), the temporal rate of volume change is very slow. This is because the volume change kinetics of these gels are usually controlled by diffusion of the polymer network through the solvent, a very slow process. For example, a sheet of PNIPAAm gel Imm thick swells or shrinks to equilibrium in a few hours. See Kabra et al., Polymer, 33: 990-995 (1992), incorporated herein by reference. Of course, volume change gels can be made very thin or as extremely small beads, on the order of microns, and the gels will respond very quickly to a change in external stimulus such as temperature. See, for example U.S. Pat. No. 5,183,879. This is because their volume change is controlled by the rate of heat transfer through the gel. However, very thin responsive gels are impractical for any uses that require strength and structural integrity, such as for example, bioreactors, drug delivery systems, mechanico-chemical actuators, and the like.
Only a few attempts have been made to synthesize larger gels having volumetric changes measured on short time scales. See, for example, Huang et al. (J. Chem. Eng. Japan, 20: 123-128, 1987) who analyzed the kinetic behavior of a polyvinylmethylether (PVME) gel made by the gamma radiation, free radical cross-linking of a PVME solution mixed with ferric oxide powder.
This radiation cross-linking method has been critically investigated by Kabra et al., supra who found that the radiation cross-linking method is problematic. Many of the PVME gels made by Kabra et al. had bubbles in them, formed from the hydrogen gases generated during gamma-irradiation. Moreover, although their gels were chemically indistinguishable from those of Huang et al., the Kabra et al. gels that showed the most consistent kinetics were still much slower than the identically made gels of Huang et al.
Significantly, the radiation cross-linking method produced some gels that displayed volume kinetics no different than conventionally-made responsive gels. Moreover, the radiation cross-linking method produced gels with pore sizes consistently greater than 10 microns.
The problems with radiation cross-linking were partially circumvented by synthesizing a microporous gel of cross-linked monomeric N-isopropylacrylamide (NIPA) using a method in which the solution temperature was increased above the lower critical solution temperature (LCST) during the polymerization/cross-linking reaction. Kabra and Gehrke, Polymer Comm. 32: 322-323, 1991. The NIPA gels made by cross-linking monomeric units in this manner displayed an absolute magnitude of volume change (ratio swollen/collapsed=2:1) that was much less than the absolute magnitude of volume change displayed by conventionally-made NIPA gels (ratio=10:1). Significantly, however, monomer reactions are difficult to control, especially during phase separation and many of the gels were structurally weak and were not self-supporting. That is, they deformed under gravity and could not maintain their structural integrity and dimensions when removed from the solvent. Formation of gels from monomeric precursors is also inappropriate for making gels from cellulose ethers, polypeptides, polysaccharides, and other natural materials.
The relationship between synthesis conditions, structure and properties of microporous responsive gels remains unexplored and the rules governing synthesis of fast response microporous gels with predictable structure and response kinetics are not well characterized. It is not yet possible to predict with any certainty if a microporous gel will be a rapid response gel. Mere knowledge that a microstructure exists is insufficient to predict that a given responsive gel will have rapid and consistent volume change kinetics.